Ultrasonic transducers require circuits for driving the same. The efficiency of an ultrasonic transducer system is, to a large extent, determined by the configuration of the drive circuit.
In U.S. Pat. No. 4,141,608, entitled "Circuitry for Driving a Non-linear Transducer for Ultrasonic Cleaning" which issued Feb. 27, 1979 to Breining, et al. and assigned to the assignee of the instant invention, it is taught that the power dissipation in the drive circuitry for an ultrasonic transducer can be reduced by providing a square wave driving signal to a resonant circuit including the ultrasonic transducer. In the circuit disclosed therein, however, the square wave is generated by a square wave generator having a frequency independent of the resonant characteristics of the ultrasonic transducer load circuitry so that the phase between the current and voltage in the drive circuitry is not controlled thereby.
In U.S. Pat. No. 3,651,352, entitled "Oscillatory Circuits for Ultrasonic Cleaning Apparatus" which issued Mar. 21, 1972 to William L. Puskas, an oscillatory circuit is disclosed for driving an ultrasonic transducer load circuit. In this circuit, a pair of resonant circuits is employed. This patent teaches that the resonant frequency of the resonant circuit connected to the drive circuitry should be a multiple even integer of the resonant frequency of the crystal transducer being driven. Thus, the current at the switching time of the transistor is not at its maximum value. In this circuit, the degree to which the current can be minimized at the switching time is dependent upon the accuracy to which the two resonant circuits can be made to be even multiples of each other and, further, even if this could be done perfectly accurately, the current value would not be zero at the switching time.
An article appeared at pages 33 through 38 in the summer, 1980 edition of R. F. Design, entitled "Class E Switching-Mode RF Power Amplifiers", which discusses the advantages of having the voltage and current transitions occur at different times in switching amplifiers to minimize power dissipation. This article also teaches that the voltage across the transistor at turn-on time should be the saturation voltage and that the slope of the transistor voltage at turn-on time should be zero. This article, however, does not teach an oscillatory driver circuit for an ultrasonic transducer, particularly one in which the current is zero at the switching time of the drive transistors.